TREM2 variant p.R47H as a risk factor for sporadic amyotrophic lateral sclerosis

Mutation analysis of patients with neurodegenerative disorders using NeuroX array

Genetic analyses of patients with neurodegenerative disorders have identified multiple genes that need to be investigated for the presence of damaging variants. However, mutation analysis by Sanger sequencing is costly and time consuming. We tested the utility of a recently designed semi-custom genome-wide array (NeuroX; Illumina, Inc) tailored to study neurodegenerative diseases (e.g., mutation screening). We investigated 192 patients with 4 different neurodegenerative disorders for the presence of rare damaging variations in 77 genes implicated in these diseases. Several causative mutations were identified and confirmed by Sanger sequencing, including PSEN1 p.M233T responsible for Alzheimer's disease in a large Italian family, as well as SOD1 p.A4V and p.I113T in patients with amyotrophic lateral sclerosis. In total, we identified 78 potentially damaging rare variants (frequency <1%), including ABCA7 p.L400V in a family with Alzheimer's disease and LRRK2 p.R1514Q in 6 of 98 patients with Parkinson's disease (6.1%). In conclusion, NeuroX appears to be helpful for rapid and accurate mutation screening, although further development may be still required to improve some current caveats.

Frontobasal gray matter loss is associated with the TREM2 p.R47H variant

A rare heterozygous TREM2 variant p.R47H (rs75932628) has been associated with an increased risk for Alzheimer's disease (AD). We aimed to investigate the clinical presentation, neuropsychological profile, and regional pattern of gray matter and white matter loss associated with the TREM2 variant p.R47H, and to establish which regions best differentiate p.R47H carriers from noncarriers in 2 sample sets (Spanish and Alzheimer's Disease Neuroimaging Initiative, ADNI1). This was a cross-sectional study including a total number of 16 TREM2 p.R47H carriers diagnosed with AD or mild cognitive impairment, 75 AD p.R47H noncarriers and 75 cognitively intact TREM2 p.R47H noncarriers. Spanish AD TREM2 p.R47H carriers showed apraxia (9 of 9) and psychiatric symptoms such as personality changes, anxiety, paranoia, or fears more frequently than in AD noncarriers (corrected p = 0.039). For gray matter and white matter volumetric brain magnetic resonance imaging voxelwise analyses, we used statistical parametric mapping (SPM8) based on the General Linear Model. We used 3 different design matrices with a full factorial design. Voxel-based morphometry analyses were performed separately in the 2 sample sets. The absence of interset statistical differences allowed us to perform joint and conjunction analyses. Independent voxel-based morphometry analysis of the Spanish set as well as conjunction and joint analyses revealed substantial gray matter loss in orbitofrontal cortex and anterior cingulate cortex with relative preservation of parietal lobes in AD and/or mild cognitive impairment TREM2 p.R47H carriers, suggesting that TREM2 p.R47H variant is associated with certain clinical and neuroimaging AD features in addition to the increased TREM2 p.R47H atrophy in temporal lobes as described previously. The high frequency of pathologic behavioral symptoms, combined with a preferential frontobasal gray matter cortical loss, suggests that frontobasal and temporal regions could be more susceptible to the deleterious biological effects of the TREM2 variant p.R47H.

Coding variants in TREM2 increase risk for Alzheimer's disease

The triggering receptor expressed on myeloid 2 (TREM2) is an immune phagocytic receptor expressed on brain microglia known to trigger phagocytosis and regulate the inflammatory response. Homozygous mutations in TREM2 cause Nasu-Hakola disease, a rare recessive form of dementia. A heterozygous TREM2 variant, p.R47H, was recently shown to increase Alzheimer''s disease (AD) risk. We hypothesized that if TREM2 is truly an AD risk gene, there would be additional rare variants in TREM2 that substantially affect AD risk. To test this hypothesis, we performed pooled sequencing of TREM2 coding regions in 2082 AD cases and 1648 cognitively normal elderly controls of European American descent. We identified 16 non-synonymous variants, six of which were not identified in previous AD studies. Two variants, p.R47H [P = 9.17 × 10(-4), odds ratio (OR) = 2.63 (1.44-4.81)] and p.R62H [P = 2.36 × 10(-4), OR = 2.36 (1.47-3.80)] were significantly associated with disease risk in single-variant analyses. Gene-based tests demonstrate variants in TREM2 are genome-wide significantly associated with AD [PSKAT-O = 5.37 × 10(-7); OR = 2.55 (1.80-3.67)]. The association of TREM2 variants with AD is still highly significant after excluding p.R47H [PSKAT-O = 7.72 × 10(-5); OR = 2.47 (1.62-3.87)], indicating that additional TREM2 variants affect AD risk. Genotyping in available family members of probands suggested that p.R47H (P = 4.65 × 10(-2)) and p.R62H (P = 6.87 × 10(-3)) were more frequently seen in AD cases versus controls within these families. Gel electrophoresis analysis confirms that at least three TREM2 transcripts are expressed in human brains, including one encoding a soluble form of TREM2.

Altered microglial response to Aβ plaques in APPPS1-21 mice heterozygous for TREM2

BACKGROUND

Recent genome-wide association studies linked variants in TREM2 to a strong increase in the odds of developing Alzheimer's disease. The mechanism by which TREM2 influences the susceptibility to Alzheimer's disease is currently unknown. TREM2 is expressed by microglia and is thought to regulate phagocytic and inflammatory microglial responses to brain pathology. Given that a single allele of variant TREM2, likely resulting in a loss of function, conferred an increased risk of developing Alzheimer's disease, we tested whether loss of one functional trem2 allele would affect Aβ plaque deposition or the microglial response to Aβ pathology in APPPS1-21 mice.

RESULTS

There was no significant difference in Aβ deposition in 3-month old or 7-month old APPPS1-21 mice expressing one or two copies of trem2. However, 3-month old mice with one copy of trem2 exhibited a marked decrease in the number and size of plaque-associated microglia. While there were no statistically significant differences in cytokine levels or markers of microglial activation in 3- or 7-month old animals, there were trends towards decreased expression of NOS2, C1qa, and IL1a in 3-month old TREM2+/- vs. TREM2+/+ mice.

CONCLUSIONS

Loss of a single copy of trem2 had no effect on Aβ pathology, but altered the morphological phenotype of plaque-associated microglia. These data suggest that TREM2 is important for the microglial response to Aβ deposition but that a 50% decrease inTREM2 expression does not affect Aβ plaque burden.